1. Field of the Invention
The present invention relates to a computer controlled power supply assembly for LED array. In particular, the present invention relates to a feedback system for regulating the DC power supply to a LED array.
2. Description of Related Art
Most, if not all textbooks make the point of saying that LEDs should not be operated in parallel unless large series resistors are used to prevent over current. Series arrays can require several hundred volts for large arrays. For example, a series array consisting of 500 LEDs conceivably requires a voltage of 1000 volts, whereas the same LEDs arranged in a parallel array could operate at 2 volts.
LED arrays are used in vehicle traffic control signal heads and other larger industrial arrays, such as those used by banks to display the temperature to passing motorists.
Manufacturers of LED arrays are using a technique called “binning” which, in addition to better quality control, has resulted in more uniform properties of LEDs. A practice used by some manufacturers is a simple series configuration of the LEDs. In this mode of operation, the failure of one LED will cause the entire series string to extinguish, and for large strings, the drive voltage can be several hundred volts which can pose a safety hazard. Combination series-parallel circuits also require large voltages and a single failure will extinguish the series string. This type of failure is readily visible in many traffic control signal heads. The increase in efficacy of LEDs can be lost when an efficient drive circuit is not employed. A wide variety of drive circuits are being used by present manufactures. An ideal LED drive circuit should be capable of driving red, green, and yellow LED arrays and directional arrows with only minor modifications to the feedback control circuitry.
Manufactures have approached maximizing the power delivery to LED arrays in a number of ways. U.S. Pat. No. 5,661,645 describes a power supply for a light emitting diode array which includes a circuit for interrupting the supply of power from the power supply to the LED array. This increases the transient currents to the LEDs reducing life. Also, included are ballast resistors in series with the LEDs to limit current. This severely reduces efficiency. The patent claims that typical power levels are 14 watts for an 8 inch traffic control signal head. U.S. Pat. No. 6,320,330 describes an electronic device suitable for only two series LEDs. U.S. Pat. No. 6,329,760 describes a circuit arrangement for pulsing or flashing a LED. This circuit is not applicable to large arrays that must be on continuously.
U.S. Pat. No. 7,071,762 describes a power supply assembly for a LED lighting module that supplies a constant current to a LED lighting module, by using low frequency pulse width modulation PWM. This circuit is inherently hazardous because of the large voltages required to drive large LED arrays. It also reduces LED life because of the high pulsed currents required to get an average current that does not exceed the manufacture's rated current. As the pulse width decreases in time, the current pulses increase in amplitude in order to maintain a constant current.
Currently, there is no system that allows the use of parallel LED arrangements without the use of large series resistors.